1. Field of the Invention
The invention concerns an adapter antenna which without contact can receive and transfer an electromagnetic wave within the near field of another antenna. The most common application of an adapter antenna is for connection of a mobile telephone which temporarily is used within a vehicle to an external antenna located on the outside of the vehicle.
2. State of the Art
All mobile telephones are provided with a built-on first antenna, but not all models are provided with a coaxial outlet for connection to an external second antenna. If one wishes to use an external antenna with a mobile telephone lacking such an antenna outlet, an adapter antenna can be located near the first antenna and connected by a coaxial cable (33) to the second antenna.
In the following will be described what is peculiar for an adapter antenna compared to other antennae in general. Antennae are usually located in the far field of a first antenna, where the wave front is approximatively flat with the electric and magnetic field vectors at right angles to each other. In this case, the antennae do not influence the current distribution of each other, and they may be designed independently of each other. An adapter antenna is located close to a first antenna in the near field of this. The electric and magnetic field vectors are here generally not at right angles to each other. The current distributions of the first antenna and the adapter antenna will influence each other. In an ideal case the transfer between the first antenna and the adapter antenna should be total, and all power from the first antenna should be collected by the adapter antenna, which in practise can never be acheived. In case of use in a vehicle, the non-transferred power will radiate into the vehicle. This is a great disadvantage, the radiation has not fully recognized biological effects which were the reason for deciding to use an external antenna. These decribed conditions, which are specific for adapter antenna compared to other types of antennae, mean that adapter antennae should be individually optimized for each type of first antenna. The optimizing may be more difficult if the telephone is of multiple N-band type, where N is larger than 1.
The most common type of antenna for mobile telephones is monopole or small helix. Adapter antella for such are known from patents U.S. Pat. No. 4,220,955, GB 2266967, SE 500983, SE 503930, U.S. Pat. No. 5,557,287, U.S. Pat. No. 5,777,585, SE 504343, and WO 98/25323. These all use a coil as a coupling element. Adapter antennae suitable also for other types of telephone antennae can be shaped as meander convolutions as described in patents SE 506726 and SE 507100.
The known types of adapter antennae are not optimal for certain types of first antennae, such as inverted L, E and F(PIFA) antennae. Coils are less suitable for adapter antennae to these. One aim of the invention is to acheive an optimal solution by utilizing the electric field component, implying acting as a condenser together with a suitable adaptive circuit. The invention can be used for single as well as multiple frequency band adapter antennae.
The optimizing is made in the following steps:
Determine the smallest condenser plate which can efficiently transfer the power. This is made by calculating or measuring the current distribution of the first antenna and determining the surface or surfaces where the current density is large, and designing the condenser plate of the adapter antenna to have a corresponding surface.
Choose feed point and means for matching the impedance of the coaxial cable, such as 50 ohm. At the other end of the coaxial cable the second antenna or a amplifier unit may be connected.
Provide a suitable ground plane, such as a loop around the condenser plate.
Optimize the adapter antenna while accounting for its influence upon the adaptation of the first antenna in its feed point.